1. Field of The Invention
The subject invention relates to a recorder or a signal transmission apparatus, and especially to a system of transmission and reception which is excellent in a signal-to-noise ratio.
2. Description of The Related Art
A system called a quadraphase modulation system has been proposed for transmission of digital signals. A signal subjected to quadraphase modulation has few low-frequency components, and therefore, it is suitable for the signal transmission in a system, such as a magnetic recording system and a communication system using metal wires, which does not allow low-frequency signals to pass through. This system will be summarized by using a functional block diagram of FIG. 2 and a waveform diagram of FIG. 3. It is assumed that data ..., a.sub.2n, a.sub.2n+1, ... are given from a signal source to an input end of a quadraphase modulation transmitter. These data are converted into parallel data of an even-number series and an odd-number series by a serial-parallel converter and modulated simultaneously by a sine modulator and a cosine modulator. When the data after modulation are denoted by ..., b.sub.4n, b.sub.4n+1, b.sub.4n+2, b.sub.4n+3, ..., the following relationships are established: EQU b.sub.4n =a.sub.2n EQU b.sub.4n+1 =a.sub.2n+1 EQU b.sub.4n+2 =-a.sub.2n EQU b.sub.4n+3 =-a.sub.2n+1 ( 1)
As is seen from these relationships, the series of even orders of b.sub.4n, b.sub.4n+2 are made to correspond to the series of an even order of a.sub.2n, while the series of odd orders of b.sub.4n+1, b.sub.4n+3 are made to correspond to the series of an odd order of a.sub.2n+1 independently of the above according to this quadraphase modulation system. When the series of the even and odd numbers of a.sub.n are modulated independently as shown in FIGS. 3b and 3c and then added up, accordingly, a waveform after being subjected to quadraphase modulation as shown in FIG. 3d is obtained. In this case, 0, +1, 0 and -1 covering a period of 2T together is a fundamental waveform as shown in FIG. 4a. According to this modulation system, in other words, every two bits of input data are put in a block and coded, and therefore, one period is composed of two bits. The series of FIGS. 3b and 3c have a phase difference of just 90 degrees from each other in relation to this period. Therefore, the former series is called sine modulation and the latter cosine modulation.
In a receiver of quadraphase modulation, first the deterioration in a frequency characteristic caused in the course of transmission is compensated by an equalizer of FIG. 2. Then, the data are binary-coded and the waveform in FIG. 3d is restored. Next, this binary-coded waveform is discriminated and regenerated at points of ..., S.sub.4n, S.sub.(4n+1), S.sub.(4n+2), ... and ..., S.sub.(4n+1), S.sub.(4n+2)+1, S.sub.(4n+1)+2, ... shown in FIG. 3e, and thereby data shown in FIG. 3f are obtained. These data are further passed through a parallel-serial converter, and thereby the original data shown in FIG. 3g are regenerated. The above is the gist of the quadraphase modulation system, and further details thereof are described in IEEE Trans. on Magnetics, Vol. MAG-15, No. 6, 1465-1467, by J. A. Bixby, etc.
The fundamental waveform for cosine modulation according to the quadraphase modulation system is 0, +1, 0 and -1 covering together the period of 2T shown in FIG. 4a. This fundamental waveform needs to be subjected to waveform equalization so as not to cause an intersymbol interference to a code series which is sine-modulated and shifted by T/2 therefrom. According to the prior art, the equalization is conducted so that an impulse response of the above-mentioned fundamental waveform has such a waveform as shown in FIG. 4b. When this impulse response is denoted by Ir(t), in other words, it is given as: EQU Ir(O)=+1 EQU Ir(-T)=-1 EQU Ir(-nT/2)=0 (2)
where n.noteq.0 and n.noteq.1.
An equalized waveform of sine modulation in relation to a fundamental wave is obtained likewise by shifting an equalized waveform of cosine modulation by T/2 as indicated by a dotted line. By the equalization stated above, the original pulse series can be discriminated and regenerated without any intersymbol interference at any points of ..., S.sub.4n, S.sub.(4n+1), S.sub.(4n+2), S.sub.(4n+3), ... as shown in FIG. 4c. For this purpose, accordingly, it is only required to select sampling points in series of (S.sub.4n, S.sub.(4n+1)), (S.sub.(4n+1), S.sub.(4n+1)+1) and others or sampling points in series of (S.sub.(4n+2), S.sub.(4n+3)), (S.sub.(4n+1)+2, S.sub.(4n+1)+3) by the procedures of FIG. 3e, 3f and 3g, for instance, and to discriminate and regenerate the data of each phase.
A frequency band necessary for realizing the waveform of FIG. 3b is determined by a pulse width of a fundamental waveform. Now, since the value of the width of the fundamental waveform is T/2 as shown in FIG. 4a, a Nyquist rate is 1/T. Since the pulse width of the original signal is T, on the other hand, the Nyquist rate in this case is 1/(2T). In other words, the quadraphase modulation system necessitates a transmission band twice as wide as NRZ. Noise increases as a required band widens, and this causes a disadvantage that the number of bit errors also increases.